Reduction of transmitted mechanical energy in the form of shock and vibration between a mass and a support, such as a vehicle body (a sprung mass) and a vehicle wheel (an unsprung mass), is a problem of considerable importance in suspension systems, cab suspensions, seat suspensions and also in the support of sensitive equipment and payloads. Such isolation systems for reducing the transmittal of shock and vibratory energy between a mass and a support are typically disposed between a mass and the support.
For purposes of this invention, prior art isolation systems will be considered as passive, active, and semi-active. Passive isolation devices such as springs or spring-damper combinations as used in most automobiles have a performance that is strictly a function of their inherent structural characteristics. Although such passive devices provide effective isolation in a certain frequency range, they are subject to amplified excitation in passing through their natural or resonant frequency range. This frequency range is determined by the spring rate of the spring and the isolated mass. Because a passive device is subject to amplified excitation at its resonant frequency, harmful effects such as damage to the isolated mass or to the passive device may occur. Further, some passive isolation systems provide adequate control of the sprung mass at the natural frequency of the suspension while imposing far too much damping force between the interconnected members at higher frequencies. Thus, the selection of damping and the amount thereof is a design compromise when using a passive device.
Active isolation systems employ an external power source, which supplies energy in a controlled manner to counteract vibrational forces and to reduce their transmission. Such active isolation systems are advantageous in that they can generate forces as a function of the vibratory condition to be controlled. However, such active systems require a large auxiliary power source and typically require additional equipment such as pumps, motors, and servo-valves, which are may not be sufficiently responsive at high operating frequencies due to the limitations of such equipment to rapidly respond to control signals. Moreover, such active systems tend to be costly and require large amounts of power to operate.
A semi-active system has the inherent limitation that it can generally only produce forces opposing motion of the supported mass; it cannot generate force in the direction of motion. Thus, the term "semi-active" refers to control systems that are limited to removing energy from a system. However, semi-active systems are capable of performance nearly equivalent to that of active systems when operated in accordance with a suitable primary control method and, more particularly, control methods which emulate a so-called "Skyhook" damper such as described in Karnopp, D.C. et al., "Vibration Control Using Semi-active Force Generators," ASME Paper No. 73-DET-122 (June 1974). Semi-active dampers and various control methods for them are disclosed in Karnopp, U.S. Pat. No. 3,807,678; Miller et al., U.S. Pat. Nos. 4,821,849, 4,838,392 and 4,898,264; Boone, U.S. Pat. No. 4,936,425; and Ivers, U.S. Pat. No. 4,887,699 all owned by the assignee of the present invention.
Semi-active dampers may be either of the "on/off" type, the "orifice setting" type, or the "force controlled" type. An "on/off" semi-active damper is switched according to a suitable control method between "off" and "on" damping states. In the "on-state" the so-called damping coefficient of the damper is of a preselected, relatively high magnitude. For purposes of this invention the term "damping coefficient" means the relationship of the damping force generated by the damper to the relative velocity across the damper, which relationship is not necessarily linear. In its "off-state" the damping coefficient of the damper is approximately zero or of some relatively low magnitude.
An orifice-setting semi-active damper is also switched during operation between an "off-state", wherein the damping coefficient is approximately zero or of some relatively low magnitude, and an "on-state". However, when an orifice-setting semi-active damper is in its "on-state," the damping coefficient thereof normally is changed between a large (theoretically infinite) number of different magnitudes. The magnitude of the damping coefficient is typically determined by the diameter setting of the valve orifice of the damper.
A "force controlled" damper, in theory, is capable of creating any desired dissipative force in the "on-state" independent of the relative velocity across the damper. This is in contrast to the above described "on/off" and "orifice setting" dampers in which the "on-state" damping force depends on the relative velocity across the damper. A force-controlled damper can either be realized by use of feedback control, or by use of pressure controlled valves. In the "off-state" the force-controlled damper will command the valve to the full-open position in which the damping coefficient is approximately zero or some relatively low value.
Although semi-active suspension systems provide substantial performance advantages over other types of systems, they are known to have problems when subjected to large, abrupt input disturbances, i.e., such as those encountered on rough terrain. Excessive suspension motions and travel can result in uncomfortable or damaging force inputs to the suspension system when the suspension reaches its end of travel (either a compressed or extended condition) so as to impact the mechanical end stops of the suspension. End-stop collisions result in degraded isolation by the suspension by significantly increasing the root-mean-square (RMS) accelerations thereof. Therefore, it should be recognized that such end-stop collisions detract from ride comfort, and place undue stress on system components thereby shortening their longevity.
Semi-active isolation systems employing a above-mentioned "Skyhook" control method or a derivative thereof, as described hereinafter in further detail, tend to increase the average range of suspension deflection to provide "smoother" ride characteristics, but under certain conditions, may actually increase the incidence of suspension end-stop collisions. This tendency is discussed in Miller, "Tuning Passive, Semi-active and Fully Active Suspension Systems," Proceedings of the 27th CDC of IEEE, Vol. 3, 1988 and in Ivers et al., "Experimental Comparison of Passive, On/Off Semi-active and Continuous Semi-active Suspensions," SAE Paper No. 892484, Dec. 7, 1989.
Of course, the incidence of suspension end-stop collisions can be reduced and even eliminated by utilizing a damper with a sufficiently high damping coefficient. However, this would defeat the performance advantages of semi-active control by unnecessarily limiting the range of suspension deflection for the given range of motion of the suspension and degrading the isolation of the vehicle.
A technical solution for reducing the incidence and severity of suspension end-stop collisions in semi-active isolation systems without degrading their performance is disclosed in Miller, et al., U.S. Pat. No. 5,276,622. In the ('622) patent a method and apparatus controls the operation of an isolation system having an adjustable damper interconnecting relatively movable members. The method and apparatus attenuate the transmission of forces therebetween in which relative movement of the members is restricted beyond a certain limit by one or more end stops. The conditions of operation of the isolation system are monitored by sensors to produce data indicative of relative displacement, relative velocity, acceleration or other conditions. Damper control signals are provided to the damper to adjust the damping characteristics thereof, as determined by the data, in accordance with both a primary control method and an override control method. The override control method alters the damper command signals as necessary to increase the damping characteristics of the damper at times when the isolation system is likely to meet or exceed the end stops. This minimizes the incidence of end-stop collisions.
In the ('622) patent, the primary control method receives data from the sensor(s) and produces primary command signal(s) to be used for the attenuation of forces between the members in accordance with the preselected instructions. Preferably, a semi-active control method simulating a hypothetical "Skyhook" damper is utilized. The end-stop override control method also receives data from the sensors for producing override command signals to be used for reducing end-stop collisions in accordance with the instructions of the override control method. Thus, the end-stop override (hereinafter "ESO") control method disclosed in the ('622) patent only generates force when approaching an end-stop.
In a preferred embodiment of the end-stop override (ESO) method described with reference to FIG. 1, the force generated is preferably a function of the instantaneous speed of approach and a distance to the end-stop. Two interlocking members 42 and 44 are respectively connected to the members 12 and 14 and schematically represent the limits of travel for the system 10. The reference letter "A" represents the extension end stop of the system 10, which is reached when the members 12 and 14 reach full extension away from each other. The reference letter "B" represents the compression end stop of the system 10, which is reached when the members 12 and 14 are in a fully compressed position. So-called snubbers 43 and 45 are respectively located at end stops B and A. The snubbers 43 and 45 are typically resilient, deformable members made of elastomeric material and serve to cushion the impact of the members 42 and 44 when engaging the end stops A and B.
The designations "X" and "Vabs" respectively denote the absolute vertical displacement and the absolute velocity of the supported member 12; it being arbitrarily indicated that these are positive when in an upper direction and negative when in a downward direction. The same sign convention in the letters "Y" and "Vin" similarly designate the absolute vertical displacement and the absolute velocity of the supporting member 14. When the system 10 is at rest, the values of X, Vabs, Y, and Vin are all zero. The designation "Xr" indicates the relative displacement between the members 12 and 14 of the system 10 and is given by the difference X-Y. When the system 10 is at rest, the relative displacement Xr is zero. The designation "+Xes" represents the relative displacement of the system 10 in full compression. The designation"-Xes" represents the relative displacement of the system 10 in full extension. It is assumed, for simplicity of illustration, that the equilibrium position is midway between end stops. The designation "Vr" represents the relative velocity of the system and is given by the difference Vabs-Vin.
A microprocessor-based controller 46 produces electronic control signals for controlling the valve 38 of the damper assembly 22 in order to select the on-state damping coefficient for optimal isolation of the supported member 12. The controller 46 operates pursuant to a control method and receives data from one or more motion sensors 48, 50, 52 and 54 associated with the members 12 and 14. The sensors 48 and 50 directly detect the instantaneous relative displacement Xr, and the instantaneous relative velocity Vr, respectively, of the members 12 and 14.
In the end-stop override (ESO) system, the data from the sensors 48 and 50 is sent via lines 56 and 58 to the controller 46. The sensor 52 detects the absolute vertical acceleration "a" of the member 12 and sends this data via line 60 to the controller 46. The acceleration data from the sensor 52 may be utilized to derive displacement, absolute velocity, and/or relative velocity data. Since the data produced by the sensors 48, 50 can also be derived from the data produced by the acceleration sensors 52 and 54, it will be appreciated by those skilled in the art that not all of the illustrated sensors need be employed in association with the system 10 at any one time.
Referring now to FIG. 2, illustrated is a functional block diagram of the (ESO) system 10 labeled PRIOR ART, showing the details of the controller 46. Suspension block 64 represents the dynamic elements of the system 10 including the members 12 and 14, the spring 20, the damper assembly 22, and the sensors 48, 50, 52, and 54. The controller 46 receives electrical signals from the sensors in the suspension block 64 indicative of the displacement, velocity, and/or acceleration of the members 12 and 14 as discussed above. The controller 46 processes the sensor data in real time using known semi-active control methods to supply damper command force signals Fc to the damper 22. The signals Fc are used to vary the amount of damping of the damper 22 in order to provide improved isolation of the support member 12. While not shown, it will be understood that the controller 46 may be embodied as analog circuitry or as a digital computing system.
In the preferred end-stop override system (ESO) shown in FIG. 2, the controller 46 includes primary control block 66, an override control block 68, and a summing device 70. The primary control block 66 implements a primary control, as discussed further below, to supply primary control command force signals Fp to the summing device 70 based upon the signals received from the suspension block 64 on one or more of the sensor lines 56, 58, 60, and 62. The override control block 68 receives from the suspension block 64 signals on the sensor lines 56 and 58 indicative of the relative velocity Vr and the relative displacement Xr of the members 12 and 14. The override control block implements a unique override control method for supplying end-stop override command force signals (Fes) to the summing device 70. The summing device 70 combines the Fp signal and the Fes signal and, using appropriate gain devices and/or other circuitry, (not shown), supplies the damper command force signal Fc to the damper 22.
The end-stop override command force signal Fes contributes to the damper command force signal Fc only at times when the primary control command force signal Fp is unable to cause the damper to generate a force necessary to avoid an impending end-stop collision. The override control block 68 only intervenes when necessary to prevent end-stop collisions, but otherwise allows the primary control block to govern operation of the system. It should be appreciated that when the ESO control is implemented it only effectuates a change in the on-state force.
The primary control block 66 is preprogrammed to operate in accordance with a standard version of any one of a plurality of semi-active damper control methods and, more particularly, with those methods, and derivatives thereof, which emulate the so-called Skyhook damper as described in Karnopp, D.C. et al., and as cited hereinabove.
The so-called Skyhook control method is based upon the sign of the product of the relative velocity Vr between the supported and supporting members 12 and 14 times the absolute velocity Vabs of the supported member 12. More specifically, the standard version of the Skyhook control method dictates that the damping coefficient of the damper be approximately zero when the product Vabs*Vr is less than zero. This is known as the "off-state" and takes place either (1) when the relative velocity Vr of the members 12 and 14 is positive, i.e., when the members 12 and 14 are separating and the velocity Vabs of member 12 is negative, i.e., downward; or (2) when the relative velocity Vr of the members 12 and 14 is negative, i.e., members 12 and 14 are coming together and the velocity Vabs of member 12 is positive, i.e., upward.
On the other hand, the standard Skyhook control method dictates that the damping coefficient of the damper 22 be proportional to the absolute velocity Vabs when the product Vabs*Vr is greater than zero. This is known as the "on-state" and takes place either (1) when the relative velocity Vr of the members 12 and 14 is positive and the velocity Vabs of member 12 is positive; or (2) when the relative velocity Vr of the members 12 and 14 is negative, i.e., members 12 and 14 are coming together and the velocity Vabs of member 12 is negative.
FIG. 3 is a block diagram which further describes a Skyhook control method of the PRIOR ART. This Skyhook control method requires two system inputs and generates a single control output to drive the controllable damper 22. The first input is the relative position Xr which is obtained by a sensor 48. The Xr signal is sent to an operating block 101, which executes the process of differentiating the relative position signal, as represented by the symbol dXr/dt, and thereby generates a relative velocity signal estimate Vr.
The second input is the absolute acceleration al, which is obtained from a sensor 52. Subsequently, an operating block 100 performs an integration of the absolute acceleration al as represented by the integral of al dt designation in the block. The absolute velocity estimate Vabs is thereby generated.
The Vabs signal is then sent to operating block 102. Operating block 102 executes the step of multiplying the absolute velocity Vabs by the relative velocity Vr and thereby generates a signal representing the value Vabs*Vr. This signal is then sent to Skyhook switch block 105. Still referring to FIG. 3, the operating block 99 represents the off-state damper signal and generates a zero value signal at all times. This zero value signal is sent to Skyhook switch block 105. Switch block 105 carries out the primary Skyhook control method. If the product Vabs*Vr is positive, switch block 105 closes the switch as at 112 causing the Vabs, which is scaled by a positive gain factor G as shown in operating block 106, to pass through the switch thereby activating the on-state of the Skyhook control method.
On the other hand, if the product of Vabs*Vr is negative, Skyhook switch block 105 closes the switch as at 114 thereby causing the zero value signal to pass through the switch activating the off-state of the Skyhook control method. Thus, either the on-state or the zero value off-state signal (whichever is applicable at a given point in time) is sent as a primary command signal Fp of the primary Skyhook control method to output 107.
Referring again to FIG. 2, it will be understood that the output command signal Fp as at 107 is then sent from the primary control block 66 to the summing device 70 based upon the signals received from the suspension block 64. In its implementation of this Skyhook control method, the primary control block 66 may obtain the necessary data with respect to the relative velocity Vr from the sensor 50 or may derive all of the necessary data from that supplied by the sensors 52 and 54 or from some other source.
Still referring to FIG. 2, the control block 68 implements an end-stop override control method which effectively overrides the aforementioned primary Skyhook control method to produce an increased damper force when necessary to avoid end-stop collisions. According to the preferred embodiment, the end-stop calculation block 72 logically determines when the instantaneous relative velocity Vr of the members 12 and 14 exceeds a predetermined maximum relative velocity value "Vr.sup.m " above which an end-stop collision may occur. The end-stop calculation block 72 generates an error value "e," which represents the difference between the instantaneous relative velocity Vr and the maximum allowable relative velocity Vr.sup.m and provides it to a feedback control block 74. The error value "e" may then be used to generate a force that is a function of the error value for feedback to the summing device 70, which generates the end-stop override command force Fes.
Thus, the combination of Skyhook control method and End-Stop Override (ESO) method would seem to provide the best of both worlds in semi-active suspension systems, that is low acceleration and fewer end-stop hits. However, when very severe inputs are exposed to the suspension system, and constraints do not allow adequate force available from the damper, jarring end-stop collisions still may occur even when ESO control method is operative. Therefore, there is a need for an improved control system whereby such extreme inputs are accommodated in such systems where the achievable damping force is limited (as in most, if not all, real world damper systems) and such end-stop collisions are avoided.